None the less, he was enlisted as an AB before he got his commision. The same thing is true of every ROTC cadet. We were enlisted in the reserve as an E-1, in a non-pay status until we got our commision. (Even if we got a scholarship, that money and our ROTC monthly stipend were not "pay" which was nice since it meant it wasn't taxed). This was so that if we failed to complete the program, they could order us to extended active duty as an enlisted airman.
In Bush's case it meant he wasn't subject to being drafted while his commission paperwork went through. They did make him go to basic training aparently, somewhat surprising since I'd expect them to send him to some form of Officer Canidate school instead. OCS is not strictly required for a Guard commision, nor for a reserve or regular commission for nurses, JAGs and doctors, although there is a shorty version even for them, IIRC.